If you summon a dog on a Multiplayer server, tame it and then do
/data remove entity [wolf UUID] Owner
then the dog should become untamed. it doesn't and i assume this is a bug in the /data remove command (unless I've completely misunderstood how the command works)
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Can confirm in 20w51a.
Can confirm in 21w03a.
Can confirm in 21w05b.
Can confirm in 1.16.5 and 21w07a.